This specification generally relates to a CMOS sensor, and more particularly to a look-ahead shutter system in such a sensor.
Complementary metal-oxide semiconductor (CMOS) active pixel sensor (APS) technology is an integrated circuit technology for electronically capturing images, therefore forming an “electronic film.” Unlike charge-coupled device (CCD) image sensors that have been used in the past to capture electronic images, the CMOS sensors use mainstream microelectronic fabrication processes to produce the sensor chips. The advantages of the CMOS APS technology are low cost, small size, and have an ability to integrate a significant amount of very-large-scale-integrated (VLSI) electronics on-chip.
In a CMOS APS device, each pixel of an image sensor array has both a photodetector and an active transistor circuitry for readout of the pixel signal. The photodetector for each pixel in the image sensor array converts photons impinging on the pixel to an electric charge via the photoelectric effect. The charge is integrated over a period of time that is long enough to collect a detectable amount of charge but short enough to avoid saturating storage elements. This period is analogous to a film exposure time (i.e. the shutter speed) and is called an integration time. In the image sensor array of the CMOS APS device, the integration time is the time between a reset of a given row and a subsequent read of the row. Since only one row can be selected at a time, the reset/read routine is sequential (i.e. row by tow). This routine is referred to as a “rolling electronic shutter.” The integration time may be adjusted by a fixed amount as a result of an image exposure analysis using an average intensity scheme or image histogram measurements. The shutter width is the time between integration enable and readout. This may be of any size depending on the number of adjacent pixels that will have the same integration time. The shutter width may also be adjusted by a fixed value to control the gain of an exposed area of a sensor array.
To realize an integration time shorter than one frame time, a particular row needs to be accessed at some given time before the readout. The accessed row needs to be reset to start a new integration. Thus, a reset pointer moves ahead of a read pointer by an amount equal to the shutter width. The time difference between the two pointers is the integration time.
However, when the integration time is adjusted dynamically, the integration process may encounter some difficulties in implementing the change. For example, when the integration time is increased, the reset pointer needs to jump to the beginning of the frame without completing the previous reset cycle. This may be seen as an artifact or a short flash in the bottom portion of the image.